SACRAMENTO — San Francisco Mayor Gavin Newsom has pulled way ahead of Los Angeles City Councilwoman Janice Hahn in the Democratic primary race for lieutenant governor, according to Newsom’s internal polling.

Newsom has a 21 percentage point lead — 47 percent to 26 — with more cash on hand and only a month to go in the primary race. The rest — one-third of the voters — were undecided.

“It’s nearly impossible for her to make serious inroads,” said Ben Tulchin, Newsom’s pollster. What could cinch the deal is his massive — 62 percent to 16 — lead in the Bay Area. “If you’re up 46 points in the biggest Democratic region of the state, (a comeback by Hahn) basically can’t be done.”

Hahn leads Newsom 42 percent to 34 percent in the Los Angeles region, but in all other areas, Newsom does better. In the Central Valley, he has a 28 percentage point lead over Hahn.

Hahn’s campaign manager, Michael Trujillo, said that he suspected that the internal poll used Newsom’s high name identity to get at its final numbers. If told positive attributes about both candidates, Trujillo said, voters would be more open to Hahn’s candidacy.

“Democratic voters aren’t exactly enamored with Gavin Newsom,” Trujillo said. “That’s why he can’t get over 47 percent against an unknown councilwoman from Los Angeles. When voters realize there’s a progressive, solid Democratic candidate against Newsom, they leave him in droves.”

Newsom’s public image appears to be the decisive factor in the lead. He is recognized by 71 percent of Democratic primary voters, compared to a 30 percent recognition for Hahn. And Newsom is favorably viewed by 48 percent of the voters, compared to a 12 percent who hold a negative view. Hahn has a 13 percent favorable rating and 3 percent negative rating.

The poll surveyed 700 likely Democratic voters and had a margin of error of 3.7 percent.

With only 27 percent undecided, it will be “extraordinarily difficult for Hahn to obtain the necessary votes to overtake Newsom,” Tulchin and Julie Lein wrote in a memo to the Newsom campaign.

Since March 17, the most recent reporting period, Newsom has raised $428,550 in contributions of $1,000 or more. Hahn has raised $103,200 since March 17.

Hahn’s campaign has begun to send out 10 million mail literature to voters, and is expected to begin its statewide TV campaign in the next couple weeks, Trujillo said.

The winner of the Democratic primary will face the winner of the Republican primary between Lt. Gov. Abel Maldonado and state Sen. Sam Aanestad. Maldonado was recently appointed to the position by Gov. Arnold Schwarzenegger and confirmed by the Legislature.

Maldonado will serve out the remainder of the term of former Lieutenant Gov. John Garamendi, who was elected to Congress last fall. The term ends at the end of the year.